Regeneration and brake management system

ABSTRACT

A regeneration and brake management system includes a prime mover, an energy storage device associated with the prime mover, and a regenerative braking system including a controller and at least one friction brake, wherein the regenerative braking system is configured to at least partially disable the at least one friction brake during regenerative braking.

BACKGROUND

The present system and method relate generally to hybrid motor vehicles,and more particularly, to a brake management system adapted toregenerate a fuel source of a hybrid motor vehicle.

Automobile manufacturers are continuously working to improve fuelefficiency in motor vehicles. Improvements in fuel efficiency aretypically directed toward reducing weight, improving aerodynamics, andreducing power losses through the vehicle powertrain. However, the needto improve fuel efficiency is commonly offset by the need to provideenhanced comfort and convenience to the vehicle operator. As an example,manually-shifted transmissions are more fuel efficient than automatictransmissions due to lower parasitic losses. The higher lossesassociated with conventional automatic transmissions originate in thetorque converter, the plate clutches and the hydraulic pump used tocontrol operation of the hydraulic shift system. However, a vastmajority of domestic motor vehicles, for example, are equipped withautomatic transmissions due to the increased operator convenience theyprovide. Recent advances in power-operated shift systems have alloweddevelopment of “automated” versions of manual transmissions, whichautomatically shift between sequential gear ratios without any inputfrom the vehicle operator. Thus, automated manual transmissions providethe convenience of a traditional automatic transmission with theefficiency of a manual transmission.

Passenger vehicle and heavy truck manufacturers are also activelyworking to develop alternative powertrain systems in an effort to reducethe level of pollutants exhausted into the air by conventionalpowertrain systems equipped with internal combustion engines.Significant development efforts have been directed to electric andfuel-cell vehicles. Unfortunately, these alternative powertrain systemssuffer from several disadvantages and, for all practical purposes, arestill under development. However, “hybrid” electric vehicles, whichinclude an internal combustion engine and an electric or hydraulicmotor, offer a compromise between vehicles powered by traditionalinternal combustion engines and full electric-powered vehicles. Thesehybrid vehicles are equipped with an internal combustion engine and anelectric or hydraulic motor that can be operated independently or incombination to provide motive power to the vehicle.

There are two types of hybrid vehicles, namely, series hybrid andparallel hybrid vehicles. In a series hybrid vehicle, power is deliveredto the wheels by the electric motor, which draws electrical energy froma generator or a battery. The engine is used in series hybrid vehiclesto drive a generator that supplies power directly to the electric motoror charges the battery when the state of charge falls below apredetermined value. In parallel hybrid vehicles, the electric motor andthe engine can be operated independently or in combination pursuant tothe running conditions of the vehicle.

Improving the efficiency of hybrid vehicles includes recouping energyspent by the electric motor. Generally, the control strategy forrecouping energy spent by the electric motor involves operating themotor in a reverse operation causing it to function as a generatorduring braking operations. However, attempting to recover spent energythrough regenerative braking presents a number of issues. First, it isnot possible or practical to recover all the braking energy at grossweights, at high speeds, or at high deceleration rates because somebraking energy has to be transferred to the brakes under thesesituations. However, it would be desirable to retrieve as much of thebraking energy as practical. Second, the amount of energy retrieved andtransferred during regenerative braking varies depending on manyvariables, some of which will change during vehicle operation, such asthe amount of wind acting on the vehicle, absorber motor torquevariations, temperature changes, age of the vehicle, grade of theterrain, weight transfers, rolling resistance, etc. Third, the requestedamount of braking (deceleration) by the driver will vary and usuallyincrease as the vehicle slows. Fourth, regeneration will unbalancetwo-wheel-drive brake systems and could cause unusual brake and tirewear. Attempting to obtain the maximum amount of energy throughregeneration generally unbalances the brake system of a two-wheel drivevehicle because regeneration provides a negative torque on the drivewheels, in addition to the braking force applied by the braking system.During regenerative braking, the non-drive wheels spin freely, onlybeing acted upon by the braking system. This uneven application ofnegative torque between the drive and non-drive wheels results in apotential for skidding the drive wheels, reducing vehicle stability.With the loss of stability, moreover, there is typically uneven tirewear and brake wear. Accordingly, there exists a need for improvedregenerative brake control systems for use in hybrid vehicles thatfacilitate an efficient, yet safe regeneration of energy.

SUMMARY

A regeneration and brake management system includes a prime mover, anenergy storage device associated with the prime mover, and aregenerative braking system including a controller and at least onefriction brake, wherein the regenerative braking system is configured toat least partially disable the at least one friction brake duringregenerative braking.

According to one exemplary embodiment, the above-mentioned regenerationand brake management system is applied to a hybrid powertrain system.When applied to a powertrain system, the present system is configured tomaximize the amount of energy accumulated through regenerative brakingby at least partially disabling the friction brakes when sufficientdeceleration may be obtained through regenerative braking. Further, thepresent system reduces drive wheel skidding, minimizing vehicleinstability and resulting uneven tire and brake wear.

Additionally, an exemplary method for operating a regenerativepowertrain system includes providing a prime mover, an energy storagedevice associated with the prime mover, a regenerative braking systemincluding a controller and at least one friction brake, and at leastpartially disabling the at least one friction brake during regenerativebraking.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present system and method will now be described, byway of example, with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic view of an exemplary hybrid powertrain system fora motor vehicle;

FIG. 2 is a schematic view of an exemplary regenerative braking systemadapted for use in the hybrid powertrain system shown in FIG. 1;

FIG. 3 is a flowchart illustrating an exemplary method for maximizingregeneration in a braking system according to one exemplary embodiment;

FIG. 4 is a graphical representation of a hybrid powertrain regenerationoperation according to one exemplary embodiment thereof;

FIG. 5 is a simple schematic diagram illustrating a control system forimplementing the method of FIG. 3 according to one exemplary embodimentthereof;

FIG. 6 is a simple schematic diagram illustrating a control system forimplementing the method of FIG. 3 according to one exemplary embodiment;

Throughout the drawings, identical reference numbers designate similar,but not necessarily identical, elements.

DETAILED DESCRIPTION

The present system and method provide an integral anti-skid, hydraulicbrake pressure intensifier, and regenerative control system. Morespecifically, the present system and method are configured to maximizethe amount of energy accumulated through regenerative braking by atleast partially disabling the friction brakes when sufficientdeceleration may be obtained through regenerative braking. Consequently,regeneration of energy is maximized while providing a requested amountof deceleration to the vehicle. Additionally, the anti-skid functions ofthe vehicle are enhanced by a controller that manages and distributesthe maximum available and acceptable kinetic energy to the storagebatteries and distributes the excess energy to the appropriate frictionbrakes. By predicting and monitoring regenerator torque based oncurrent, combined with the driver's pedal input request and wheel speedinformation detection, regeneration is maximized without causing vehicleoperating problems, such as wheel slippage, imbalance that can causedrive wheel skidding and vehicle instability, resulting in uneven tireand brake wear.

Referring now to FIG. 1, a hybrid powertrain system (20) configured tobe associated with a hybrid vehicle is shown in accordance with anembodiment of the present system and method. In the illustratedembodiment, powertrain system (20) includes a first prime mover (22),such as a spark-ignited or compression-ignited internal combustionengine, and a hybrid transmission (24) that includes a second primemover in the form of a motor (26), such as an electric motor/generatoror hydraulic motor/pump. A main synchronizing clutch (28) is positionedbetween first prime mover (22) and hybrid transmission (24) toselectively engage and disengage the first prime mover (22) from thehybrid transmission (24). The main synchronizing clutch (28) may be anynumber of clutches currently known in the art such as a hydraulically orelectrically operated friction clutch.

Additionally, as illustrated in FIG. 1, the hybrid powertrain system(20) includes a regenerative braking system (40) having at least onefriction brake (42) associated with each wheel of a hybrid vehicle. Thefriction brakes (42) may be any type of friction braking mechanismincluding, but in no way limited to, hydraulically actuated brakes,electrically actuated brakes, mechanically actuated brakes, disk brakes,drum brakes, anti-lock brakes, or any other device or combination ofdevices used to selectively reduce the kinetic energy associated with amoving vehicle. Further details of the brake system (40) will be givenbelow with reference to FIG. 2.

Continuing with reference to FIG. 1, the powertrain system (20) mayinclude an electronic control unit (ECU) (30) for controlling operationof first prime mover (22), main clutch (28), brake system (40), andhybrid transmission (24). In a particular configuration, the ECU (30)includes a programmable digital computer that is configured to receivevarious input signals, including without limitation, the operatingspeeds of the first and second prime movers (22, 26), the transmissioninput speed, the selected transmission ratio, the transmission outputspeed, vehicle speed, and the friction brake actuation commands. Oncereceived, the ECU (30) processes the signals according to logic rules tocontrol operation of the powertrain system (20). For example, the ECU(30) may be programmed to deliver fuel to the first prime mover (22)when it functions as an internal combustion engine. To support thiscontrol, each of the first prime mover (22), the main clutch (28), thebrake system (40), and the hybrid transmission (24) may include its owncontroller (32, 34, 35, 36) respectively. According to one exemplaryembodiment, the brake system controller (35) is a brake managementregeneration anti-skid controller configured to implement the presentsystem and method as explained in detail below with reference to FIGS. 3and 4. However, it will be appreciated that the present system andmethod are not limited to any particular type or configuration of ECU(30), controllers (32, 34, 35, 36), or to any specific control logic forgoverning operation of the hybrid powertrain system (20).

In the exemplary embodiment illustrated in FIG. 1, the powertrain system(20) also includes at least one energy storage device (38A, 38B) forproviding energy to operate the first and second prime movers (22, 26).For example, an energy storage device (38A) that is in fluidcommunication with the first prime mover (22) may contain a hydrocarbonfuel when the first prime mover (22) functions as an internal combustionengine. In another example, the energy storage device (38B) may includea battery, a bank of batteries, or a capacitor when the second primemover (26) functions as an electric motor/generator. When so configured,the electric motor/generator may be provided in electrical communicationwith the electrical storage device (38B) through a drive inverter (39),as is known in the art. Alternatively, the energy storage device (38B)may function as a hydraulic accumulator when the second prime mover (26)functions as a hydraulic motor/pump. In the hybrid transmission (24)illustrated in FIG. 1, negative torque may be used to drive rotation ofthe second prime mover or motor (26), operating as a generator or apump, to create and store energy in the energy storage device (38B).Moreover, engine braking may be emulated, which may be desirable even ifenergy storage device (38B) is at capacity. For ease of explanationonly, the present system and method will be described hereafter in thecontext of a second prime mover or motor (26) that may function as agenerator. However, it will be appreciated that the present system andmethod may also be applied to a hydraulically driven motor configured tooperate as a pump.

With reference to FIG. 2 of the accompanying drawings, the componentsand method of the exemplary brake system (40) will now be described indetail. According to one exemplary embodiment illustrated in FIG. 2, theexemplary brake system (40) includes a brake pedal (100) that ismechanically coupled to a master cylinder (105), including a fluidreservoir. As a vehicle operator maneuvers the vehicle, desireddeceleration is input to the exemplary brake system (40) by actuation ofthe brake pedal (100). Force generated by actuation of the pedal (100)is transferred to the master cylinder (105) where it is converted from amechanical motion to a hydraulic pressure. This hydraulic pressure isfurther applied to an intensifier (110) that amplifies the hydraulicpressure generated by the master cylinder (105) as it is applied to thebrake line (135). Reliable amplification of the hydraulic pressure bythe intensifier (110) is performed with the aid of a charge pump (120)and a fluid accumulator (115), as is currently known in the art.

As illustrated in the exemplary embodiment of FIG. 2, the brake lines(135) exit the intensifier (110) and lead to a front and rear proportionvalve (130, 131) respectively. The proportioning valves (130, 131)regulate the hydraulic pressure that is eventually transferred to thefriction brakes (152-158). Regardless of the type of friction brakes thehybrid vehicle possesses, the rear brakes (156, 158) generally requireless force than the front brakes (152, 154). In other words, the amountof brake force that can be applied to a wheel without overcoming therolling friction depends, at least in part, on the amount of weight onthe wheel. Higher weight means more brake force can be applied to reducethe kinetic energy without locking the wheel up. The front brakes (152,154) generally support a majority of the weight of at least the firstprime mover (22) and the hybrid transmission (24), and consequentlydemand higher braking force or pressure. As a result, a front-wheeldriven vehicle will have the potential to regenerate more energy than arear-wheel driven vehicle. Consequently, the present exemplaryembodiment will be described in the context of a front-wheel drivenvehicle having a hybrid powertrain system (20). However, the presentsystem and method may be applied to any vehicle incorporating a hybridpowertrain system (20).

Continuing with the brake system (40) illustrated in FIG. 2, the frontand rear proportion valves (130, 131) each transmit a pressurized fluidto the inlet valves (145) of the friction brakes (152-158) with a shockdamper (140) therebetween. As the pressurized fluid is selectivelypassed through the brake inlet valves (145), it acts upon the frictionelement (not shown) of the friction brakes (152-158), thereby actuatingthe brakes and reducing the kinetic energy of the vehicle. The kineticenergy of the vehicle is reduced at a rate proportional to the pressureof the pressurized fluid acting upon the friction element of the brakes(152-158). The inlet valves (145) maintain the pressure of thepressurized fluid against the brakes (152-158) until pressure in thebrake lines (135) is reduced. Once pressure in the brake lines (135) isreduced, the brake outlet valves (150) are actuated allowing thepressurized fluid to be exhausted and flow through a return path of thebrake lines (135) to the charge pump (120) where the fluid may be reusedin the brake system (40). When the pressurized fluid is exhausted fromagainst the brakes (152-158), the braking pressure on the brakes(152-158) is reduced or eliminated.

FIG. 2 also illustrates a number of one-way valves (160) and anaccumulator (165) disposed in the brake system (40). As shown, theone-way valves (160) are oriented to permit the flow of a highpressurized fluid to a lower pressurized fluid in a first direction whena pressure differential exists in the brake lines (135). However, in asecond direction, the one-way valves (160) are configured to preventsuch a flow. Consequently, the one-way valves (160) provide astabilization of the pressurized fluid in one direction whilemaintaining a high pressure accumulation in a second direction.Similarly, the accumulator (165) disposed in the brake system (40) iscoupled to the output of the intensifier (110) and the input of thebrake inlet valves (145) through a number of one-way valves (160). Asillustrated, the accumulator (165) and the one-way valves (160) regulatea minimum pressure at the brake inlet valves (145) by allowing a flow ofpressurized fluid to the brake inlet valves when the pressurized fluidprovided by the proportioning valves (130, 131) drops below the pressureoutput by the accumulator.

Additionally, as illustrated in FIG. 2, the exemplary brake system (40)includes wheel speed pick-ups (151) and a number of pressure transducers(125) selectively placed in fluid communication with the brake lines.The pressure transducers (125) are configured to monitor the hydraulicpressure present in various portions of the brake lines (135) and relaythe resulting pressure measurements to the brake system controller (35).Similarly, the wheel speed pick-ups (151) are configured to monitor thevelocity of the vehicle wheels and convert the measured velocities to acorresponding electrical signal that may then be transferred to thebrake system controller (35; FIG. 1) or the ECU (30; FIG. 1). The brakesystem controller (35; FIG. 1) or the ECU (30; FIG. 1) may then use thereceived measurements collected by the pressure transducers (125) andthe wheel speed pick-ups (151) to further control the fluid pressureregulating components of the brake system (40). Further details of thecontrol executed by the brake system controller (35; FIG. 1) or the ECU(30; FIG. 1) will be described below with reference to FIGS. 3 and 4.

As illustrated in FIGS. 3 and 4, the present exemplary method begins byfirst determining whether a deceleration has been requested (step 300).According to one exemplary embodiment, throttle pedal (100; FIG. 2)position may be used as pre-indicator of an opportunity fordeceleration, and thus, regeneration. Additionally, a request fordeceleration and regeneration may be indicated by an increase in thefluid pressure of the brake system (40), as detected by any of thepressure transducers (125), or by activation of a brake light system.

Once a deceleration has been requested (YES, step 300) and detected bythe brake system controller (35; FIG. 1), the ECU (30; FIG. 1) or thebrake system controller determines the deceleration torque requested(step 310) by the vehicle operator. According to the present exemplaryembodiment, the pressure transducers (125; FIG. 2) may be used todetermine the deceleration torque requested from the user by correlatingthe pressure generated in the braking system (40; FIG. 2) with a desireddeceleration torque. Additionally, according to another exemplaryembodiment, the deceleration torque requested is determined from ananalysis of the amount of force exerted by the vehicle operator on thebrake pedal (100; FIG. 2).

As illustrated in FIG. 4, line F represents an exemplary hydraulicpressure generated by a vehicle operator. The pressure generated by avehicle operator's actuation of the brake pedal (100; FIG. 2) may varyanywhere from zero to over 2000 psi. As mentioned previously, thebraking or deceleration torque requested is then determined by the brakesystem controller (35; FIG. 2) or the ECU (30; FIG. 2) from thehydraulic pressure generated, as represented in FIG. 4 by line G.According to the exemplary embodiment illustrated in FIG. 4, thedeceleration torque requested (G), when applied to the wheels, willproduce a desired level of deceleration (D). The deceleration torqueused to produce the desired level of deceleration (D) may come from anycombination of regeneration braking, rolling resistance, windresistance, front friction brake application, and/or rear friction brakeapplication. As will be described below, the exemplary methodillustrated in FIG. 3 manages the allocation of the desired level ofdeceleration between various braking sources to maximize the amount ofenergy produced through braking regeneration.

Returning again to FIG. 3, the braking system (40: FIG. 2) applies thefriction brakes to an initial predetermined pressure level (step 305)and then the system determines the additional requested decelerationtorque (step 310). Line A of FIG. 4 represents the deceleration torqueprovided by an initial predetermined pressure level that acts on aportion of the wheels of a vehicle when the brakes are applied,according to the illustrated embodiment. The brakes (152-158; FIG. 2)are applied to an initial predetermined pressure level to ensure thatall the clearances between the friction causing device, such as a brakepad, and a portion of the vehicle wheel, such as a disk or drum areremoved in preparation for further application. The predeterminedpressure level of the brakes (152-158; FIG. 2) may vary, based on anumber of factors including, but in no way limited to, weight of thevehicle, operating speed of the vehicle, etc.

After the initial application of the friction brakes (step 305) isperformed, the ECU (30) or braking system controller (35) determineswhether further deceleration is requested, in excess of the decelerationprovided by the initial application of the friction brakes, to achievethe desired level of deceleration (step 315). If the initial applicationof the friction brakes is sufficient to achieve the desired level ofdeceleration and no further braking is desired (NO, step 315), thebraking system (40) continues to monitor for further decelerationrequests (step 300).

If, however, deceleration in excess of the predetermined pressure valueis requested (YES, step 315), the ECU (30; FIG. 1) and/or braking systemcontroller (35; FIG. 1) prevents further friction braking (step 320) andthe regenerator is applied (step 325). As shown in FIG. 4, the initialdeceleration torque (A) generated when the brakes (152-158; FIG. 2) areinitially applied to a predetermined pressure level is not sufficient toachieve the desired level of deceleration (D). Consequently, a quantityof deceleration is provided through regeneration as illustrated by thearea E in FIG. 4. According to the present exemplary embodiment, furtherfriction braking by the brakes (152-158; FIG. 2) beyond the initialpredetermined pressure level represented by line A is prevented tomaximize the amount of energy obtained through regenerative braking.

As mentioned previously, application of the regenerator includes,referring to FIG. 1, disengaging the main synchronizing clutch (28) todecouple the first prime mover (22) from the hybrid transmission (24).Once the first prime mover (22) is decoupled from the hybridtransmission (24), the second prime mover (26) is operated as anelectric generator. When so configured, the electric generator isprovided in electrical communication with the electrical storage device(38B) through a drive inverter (39). When the regeneration process isinitiated, further deceleration is provided to the vehicle as negativetorque that is then used to drive rotation of the second prime mover(26), operating as a generator or a pump, to create and store energy inthe energy storage device (38B). Driving the rotation of the secondprime mover (26) with negative torque of the hybrid transmissioneffectively reduces the speed of the vehicle without wasting the kineticenergy in the brakes, emulating engine braking (152-158).

Returning again to FIGS. 3 and 4, the ECU (30; FIG. 1) or braking systemcontroller (35; FIG. 1) determines, during the regeneration process,whether the system regeneration limits have been met (step 330). Thedetermination of whether the system regeneration limits have been met(step 330) is based, according to one exemplary embodiment, on theinputs received from the pressure transducers (125; FIG. 2), the wheelspeed pick-ups (151; FIG. 2), and/or the energy storage device (38B;FIG. 1). As used in the present specification, the term “systemregeneration limits” is meant to be understood as including anycondition that demands reduction or elimination of the regenerativebraking including, but in no way limited to, filling of the energystorage device (38B; FIG. 1) to capacity, approaching traction limits ofthe driven wheels, and the like.

According to one exemplary embodiment, the ECU (30; FIG. 1) and/orbraking system controller (35; FIG. 1) determines whether theregeneration limits are met (step 330) while factoring in system inputsthat may vary during operation, and consequently, may vary whetherconditions such as traction limits of the driven wheels are met. Systeminputs that may be considered by the ECU (30; FIG. 1) and/or brakingsystem controller (35; FIG. 1) include, but are in no way limited to,grade, rolling resistance, surface condition changes, weightdistributions and variances, absorbing generator torque variations, andrequested deceleration rates. According to the present system andmethod, the system inputs are automatically sensed and transferred tothe ECU (30; FIG. 1) and/or braking system controller (35; FIG. 1). TheECU (30; FIG. 1) and/or braking system controller (35; FIG. 1) thencompensate for changes in the system inputs with the regenerativebraking to maintain brake balance between the wheels and preventskidding conditions, such that the compensations are undetectable by thevehicle operator.

As illustrated in FIG. 4, the amount of deceleration torque provided bythe regenerative braking is represented by the area E. Further, point Iis the maximum deceleration torque obtainable from regenerative brakingin combination with the initial predetermined pressure level representedby line A without exceeding the system regeneration limits, according tothe present exemplary embodiment. When the regenerative braking has beenengaged, a balance is maintained between maximizing the energy generatedfrom regenerative braking and decelerating the vehicle.

If the requested deceleration (D) has not been achieved and the systemregeneration limits have not yet been met (NO, step 330), the ECU (30;FIG. 1) and/or braking system controller (35; FIG. 1) returns to aprevious state and awaits further deceleration requests (step 315) sothat the regeneration may be further applied (step 325). According tothe present exemplary embodiment, the friction brakes continue to bemaintained at the initial predetermined pressure level represented byarea A to prevent further deceleration by the friction brakes. Thiscontrol method gives priority to regenerative braking to avoid wastingkinetic energy. More specifically, the friction brakes (152-158; FIG. 1)are disabled from further application until the system regenerationlimits have been met.

If, however, the ECU (30; FIG. 1) or braking system controller (35;FIG. 1) determines that the regeneration limits of the hybrid powertrainsystem (20; FIG. 1) have been met (YES, step 330), the ECU (30; FIG. 1)or braking system controller (35; FIG. 1) enables further frictionbraking, starting with braking of the driven or front-wheel brakes first(step 335) followed by application of the rear wheel or non-drivenbrakes (step 340).

Returning again to FIG. 4, point B illustrates the point when pressurebegins to be applied to the driven or front wheel brakes (152, 154; FIG.2), providing deceleration to the vehicle in addition to thedeceleration provided by the regenerative braking (E). As shown, line Jillustrates a deceleration torque provided when pressure is applied tothe driven or front wheel brakes (152, 154; FIG. 2). Similarly, point Cillustrates the point in time when pressure is also applied to the rearwheel or non-driven brakes (156, 158; FIG. 2). Line K illustrates adeceleration torque provided when pressure is applied to the rear wheelor non-driven brakes (156, 158; FIG. 2). As mentioned previously, thepoint of operation when the regeneration limits have been met may varydepending on a number of system inputs. That is, points B and C of FIG.4 can occur anytime after the friction brakes have been set to theinitial predetermined pressure level as represented by line A, or theymay not occur at all, depending on brake requests, vehicle weight,surface condition and many other variable system inputs. Once theregeneration limit (I) has been met, whether it be because the energystorage device (38B; FIG. 1) is filled to capacity or the regenerationtorque exceeds the available traction of one or more of the wheels,further energy will then be absorbed by the friction brakes (152-158;FIG. 2). As illustrated in FIG. 4, a quantity of deceleration (H) isprovided by application of the front wheel brakes (152, 154; FIG. 2) toachieve the requested deceleration (D).

Point C illustrates a point in time when the non-driven rear wheelbrakes (156, 158; FIG. 2) begin to engage and aid in decelerating thevehicle by providing the deceleration torque represented by line K,according to the present exemplary embodiment. Engagement of the rearwheel brakes (156, 158; FIG. 2) will occur when the ECU (30; FIG. 1) orbraking system controller (35; FIG. 1) detects that maximum regenerationcurrent (I) is achieved or no rear wheel slippage is occurring or thethermal limit of the regenerative braking system (40; FIG. 1) is beingexceeded, the energy storage device (38B; FIG. 1) cannot accept any moreenergy, the front brakes are being applied, and yet the vehicle operatoris requesting still a higher level of braking. As illustrated in FIG. 4,the front brakes (152, 154; FIG. 2) and the rear brakes (156, 158; FIG.2) may be selectively applied to provide additional deceleration whilemaintaining wheel balance.

FIGS. 5 and 6 illustrate a number of exemplary control systemsconfigured to implement the above-mentioned method according to variouslevels of complexity. FIG. 5 is a simple schematic diagram illustratinga control system (500) for implementing the method of FIG. 3, accordingto one exemplary embodiment. As illustrated in FIG. 5, the ECU (30)includes a plurality of control modules and signal modifying units thatperform the exemplary method of FIG. 3. As shown, system monitoringsignals may be generated by the pressure transducers (125) and receivedby a signal conditioning unit (510), which converts the pressuretransducer signals into control signals. Similarly, the wheel speedpick-up units (151) generate wheel velocity monitoring signals, whichare received by a wheel speed conditioning module (540) and used togenerate control signals. As illustrated, the output of both thepressure signal conditioning unit (510) and the wheel speed conditioningunit (540) are combined and used to generate the regeneration commandthat is transmitted to the motor controller (36), as discussedpreviously.

Continuing with reference to FIG. 5, the motor controller (36) providesregenerator current and transmission ratio signals to the transmissionratio modifier module (550) to allow for a determination of properfriction braking based on the amount of regeneration braking performedand the condition of the variable system inputs, as previouslymentioned. Once a friction braking determination is made, based onsystem conditions, the braking control signal is combined with both theconditioned pressure transducer signal and the conditioned wheel speedsignal and provided to proportional valve control modules (520, 530) forthe front (130) and rear (131) proportion valves respectively to allowfor balancing of the driven and non-driven wheels. Accordingly, the ECU(30) provides for the maximization of regenerative braking, based on thepressure transducer signals and the wheel speed signals, prior to theoperation of the friction brakes (152-158; FIG. 2), as previouslymentioned. Additionally, according to one exemplary embodiment, thefront (130) and rear (131) proportion valves are configured to open at anominal pressure of perhaps 300 PSI to ensure brake availability in theevent of an electrical failure.

Further, as illustrated in FIG. 5, the motor controller (36) is coupledto an anti-lock braking system (ABS) (560). According to the presentexemplary embodiment, the motor controller (36) is configured torecognize requested deceleration rates that would suggest a loss oftraction. Accordingly, if the rear wheels of the hybrid vehicle losetraction, the ABS (560) will reduce the regeneration torque, and ifadditional braking is requested, the friction brakes (152-158; FIG. 2)will be applied and blended in with the pedal application force, whichis being determined and controlled by the vehicle operator. If wheeltraction is still lost, the ABS (560) will operate normally as is wellknown in the art. By retaining the functionality of the standard ABS(560), the regenerative braking may be maximized until the ABSdetermines that the wheels are likely to lose traction.

Similarly, FIG. 6 illustrates an alternative control system (600)configured to modulate friction brake application pressures in responseto a regenerative braking current. In contrast to the control systemillustrated in FIG. 5, the control system configuration (600) shown inFIG. 6 does not provide pressure and/or wheel speed signals to theproportion valves (520, 530) in an attempt to balance the application ofthe friction brakes (152-158; FIG. 2). Rather, the regeneration systemis applied and controlled by brake pressure as sensed by the brakepressure transducers (125; FIG. 2). According to the exemplary controlsystem configuration (600), the friction brakes (152, 158; FIG. 2) aredisabled, as mentioned above, as long as the regenerative braking systemcan provide the desired deceleration (braking) range requested by thevehicle operator, and provided that the wheels do not break traction asdetected by the ABS system. According to the exemplary control systemconfiguration (600) illustrated in FIG. 6, brake light signals (610),combined with brake pressure transducer (125) signals are used totrigger a deceleration request. Once deceleration is requested, themotor controller (36) performs regenerative braking while disabling thefriction brakes (152-158; FIG. 2), according to the previously mentionedmethod, until the regeneration limits have been met and additionalfriction braking is desired. Once the regeneration limits have been met,a signal is transmitted to the proportional valve control units (520,530) to control further actuation of the friction brakes (152-158; FIG.2).

While the features of the present system and method are particularlysuited for electrically powered motors, it is possible to apply thepresent systems and methods to regenerate air systems, hydraulicsystems, or mechanical systems. Additionally, while the present systemand method were described above in the context of a front-wheel drivevehicle, the present systems and methods may be applied to vehicleshaving any number of drive configurations that fully or partiallydisable friction braking to maximize regenerative braking including, butin no way limited to a rear-wheel drive or a four-wheel drive vehicle.

In conclusion, the present system and method maximize the amount ofenergy accumulated through regenerative braking by preventing frictionbrake application in excess of an initial predetermined applicationlevel when sufficient deceleration may be obtained through regenerativebraking. Regenerative braking energy is maximized while providing arequested amount of deceleration to the vehicle without causing vehicleoperating problems, such as wheel slippage, imbalance that can causedrive wheel skidding and vehicle instability, resulting in uneven tireand brake wear.

The present exemplary system and method have been particularly shown anddescribed with reference to the foregoing embodiments, which are merelyillustrative of the best modes presently established for carrying outthe system and method. It should be understood by those skilled in theart that various alternatives to the embodiments of the system andmethod described herein may be employed in practicing the system and/ormethod, without departing from the spirit and scope thereof as definedin the following claims. It is intended that the following claims definethe scope of the system and method and that the systems and methodswithin the scope of these claims and their equivalents be coveredthereby. This description of the system and method should be understoodto include all novel and non-obvious combinations of elements describedherein, and claims may be presented in this or a later application toany novel and non-obvious combination of these elements. Moreover, theforegoing embodiments are illustrative, and no single feature or elementis essential to all possible combinations that may be claimed in this ora later application.

1. A method of operating a regenerative powertrain system comprising:providing a prime mover; providing an energy storage device associatedwith said prime mover; providing a regenerative braking system includinga controller and at least one friction brake; and at least partiallydisabling said at least one friction brake during regenerative braking;wherein said at least partially disabling said at least one frictionbrake during regenerative braking includes: detecting a vehicledeceleration request; applying said at least one friction brake to aninitial non-zero pressure level prior to said regenerative braking;initiating regenerative braking while maintaining said at least onefriction brake at said initial non-zero pressure level; and disablingfurther friction brake application during said regenerative braking. 2.The method of claim 1, wherein said initiating regenerative brakingcomprises: providing negative torque to said prime mover; and operatingsaid prime mover as a generator.
 3. The method of claim 1, furthercomprising enabling friction braking when a regeneration limit of saidregenerative braking system is met.
 4. The method of claim 3, whereinsaid enabling friction braking includes selectively applying a frictionbrake to balance said braking system.
 5. The method of claim 1, furthercomprising: monitoring said regenerative powertrain system for a wheelslippage; and operating an anti-lock braking system in response to adetected wheel slippage.
 6. A method for operating a regenerativepowertrain system comprising: providing a prime mover; providing anenergy storage device associated with said prime mover; providing aregenerative braking system including a controller and at least onefiction brake; monitoring said regenerative powertrain system for adeceleration request; determining a brake torque associated with saiddeceleration request; at least partially disabling said at least onefriction brake; and initiating a regenerative braking operation; whereinsaid at least partially disabling said at least one friction brakeincludes applying said at least one friction brake to an initialnon-zero pressure level prior to said regenerative braking operation,and disabling further friction brake application during saidregenerative braking operation.
 7. The method of claim 6, furthercomprising: disposing a pressure transducer on said regenerative brakingsystem, said pressure transducer being configured to detect a fluidpressure in said regenerative braking system; wherein said decelerationrequest includes one of a pressure increase in said regenerative brakingsystem or a brake light signal.
 8. The method of claim 7, furthercomprising: monitoring said regenerative braking system for aregeneration limit; and enabling friction braking by said at least onefriction brake when said regeneration limit is met.
 9. The method ofclaim 8, further comprising: disposing a wheel speed pick-up adjacent toa vehicle wheel, said wheel speed pick-up being configured to sense arotational velocity of said vehicle wheel; wherein said limit of saidregenerative braking system is determined from one of a regenerativecurrent, a detected fluid pressure, or a sensed wheel velocity.
 10. Themethod of claim 8, wherein said regeneration limit comprises one of anenergy storage device operating at capacity, a thermal limit of saidregenerative braking system, or a wheel slippage.